[1] : The Collaborative International Dictionary of English v.0.48

plumb \plumb\ plu^m, n. f. plomb, l. plumbum lead, a leaden
   ball or bullet; cf. gr. mo`lybos, mo`libos, mo`lybdos. cf.
   plummet, plunge.
   a little mass or weight of lead, or the like, attached to a
   line, and used by builders, etc., to indicate a vertical
   direction; a plummet; a plumb bob. see plumb line, below.
   1913 webster

   plumb bob. see bob, 4.

   plumb joint, in sheet-metal work, a lap joint, fastened by
      solder.

   plumb level. see under level.

   plumb line.
   a the cord by which a plumb bob is suspended; a plummet.
   b a line directed to the center of gravity of the earth.

   plumb rule, a narrow board with a plumb line, used by
      builders and carpenters.
      1913 webster
see also:
plummet plunge plumb line plumb bob bob plumb joint 
plumb level level plumb rule 

[2] : WordNet (r) 2.0

plumb line
     n : a cord from which a metal weight is suspended pointing
         directly to the earth's center of gravity; used to
         determine the vertical from a given point syn: perpendicular
see also:
perpendicular 

Bar Line >> B Words
Bar Line, definition of term: Bar Line
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Air Line >> A Words
Air Line, definition of term: Air Line
air+line_pag1.html

Agate Line >> A Words
Agate Line, definition of term: Agate Line
agate+line_pag1.html

Aclinic Line >> A Words
Aclinic Line, definition of term: Aclinic Line
aclinic+line_pag1.html